U.S. Pat. No. 4,861,644 describes printing upon microporous materials which comprise (1) matrices comprising large proportions of ultrahigh molecular weight (UHMW) polyolefin, (2) large proportions of siliceous filler distributed throughout the matrices, and (3) interconnecting pores. Such microporous materials accept inks well and the inks set very quickly which is especially important for high speed multipress printing runs. The printed microporous materials are durable and substantially waterproof.
It has been noticed that sometimes during the multipress printing of these microporous materials, problems with registration of the images have occurred. Although it is desired not to be bound by any theory, it is believed that the misregistrations are principally due to deformations of the microporous sheets during printing. Such deformations are believed to be the result of applied stresses such as various compressions and/or tensions occurring during movement of the microporous material through the presses.
In the context of converting polymer films, D. H. Fishman, Paper, Film & Foil Converter, July 1990, pages 45-46, indicates that the maximum stretching that can be tolerated in a film and still hold register has been determined to be 0.63%. Irrespective of the precise value, it is clear that in the case of microporous materials elongations resulting from applied stresses in printing presses should be low.
Insofar as microporous materials are concerned, the prior art has been primarily concerned with physical properties at or near the break point. The elongations at these points are usually high, and the physical properties so determined usually provide little or no information as to the magnitude of applied stresses at small elongations. Although two materials may have the same ultimate tensile strengths or the same tensile strengths at break, the paths on the stress vs. strain diagram leading to these values may be markedly different.
Good registration during printing is a result of high dimensional stability of the microporous sheet. A major element of high dimensional stability is low elongation. Therefore, it is desirable to use microporous materials which are resistant to strain at low stress. Restated, it is desirable that the slope of the curve on the stress vs. strain diagram to be very steep at low stresses.